Apparatus for making a stereolithographic object, methods for making a stereolithographic object, a method for locating the position of debris, and a method for monitoring consumption of a material for making a stereolithographic object

ABSTRACT

Disclosed herein is an apparatus (100) for making a stereolithographic object (122). Also disclosed herein are methods for making a stereolithographic object (122), a method for locating the position of debris, a method for characterising the viscosity of the material (104), and a method for monitoring consumption of a material (104) for making a stereolithographic object (122).

TECHNICAL FIELD

The disclosure herein generally relates stereolithography, andparticularly but not exclusively to apparatus for making astereolithographic object, methods for making a stereolithographicobject, a method for locating the position of debris, and a method formonitoring consumption of a material for making a stereolithographicobject.

BACKGROUND

An object can be made one section at a time, that is layerwise, using anapparatus for making an object using a stereolithographic method. In astep of the stereolithographic method, a layer of a material used formaking the object may be solidified in the shape of a section of theobject. The step may be repeated until each of a plurality of sectionsconstituting the object are made.

The position of the object being made by an apparatus, however, may notbe at a target position because apparatus generated forces deform theapparatus. This may result in inconsistent layer thickness. Compensationfor the deformation may result is more time than desired being taken toform a layer of the material of the correct thickness forsolidification.

The material may be consumed before the object is completed, in whichcase the portion of the object fabricated may need to be discarded, thematerial replenished, and the portion of the object fabricated a secondtime.

Debris may interfere with fabrication of the object and may damage theapparatus. The debris may be one of, for example:

-   -   be hardened material resulting from detachment of a partially        formed object    -   material for making the object that been unintentionally        hardened    -   foreign matter.

The unintentional hardening may be due to stray material solidifyingradiation generated by the apparatus or otherwise, which is common andproblematic.

It may be desirable to have improved apparatus for making an object.

SUMMARY

Disclosed herein is an apparatus for making a stereolithographic object.The apparatus comprises a platform for making the stereolithographicobject thereon and a material receiving surface, wherein in use amaterial for making the stereolithographic object is disposedtherebetween. The apparatus comprises a positioner operationally coupledto at least one of the platform and the material receiving surface, andoperable to change the distance between the platform and the materialreceiving surface. The apparatus comprises a force sensing systemconfigured to generate force information indicative of a forcetransmitted between the platform and the material receiving surface. Theapparatus comprises a control system arranged to generate distanceinformation indicative of the distance between the platform and thematerial receiving surface using the force information.

In an embodiment, the control system is configured to control thepositioner using the distance information. The control system may beconfigured to control the positioner to reduce the distance between theplatform and the material receiving surface using the distanceinformation.

In an embodiment, the force comprises a material displacement forcegenerated by the positioner for displacing a portion of the materialwhen so disposed to reduce the distance between the platform and thematerial receiving surface.

In an embodiment, the control system is configured to generate thedistance information by correcting for a distance information errorcaused by deformation resulting from the force.

An embodiment comprises at least one member operationally coupled to theplatform, the distance information error being caused by deformation ofat least one member by the force.

In an embodiment, the at least one member comprises a mechanical linkagebetween the platform and the material receiving surface.

An embodiment comprises memory in which is stored distance errorcorrection information, which is used by the control system to correctthe distance information error.

In an embodiment, the control system is configured to execute a methodfor generating the distance error correction information.

In an embodiment, the control system is configured to execute a methodfor empirically generating the distance error correction information.The method may comprise the steps of: engaging the platform with a stop;and generating force information indicative of a force transmittedbetween the platform and the stop for each of a plurality of positionerpositions. The force sensing system may be used to generate the forceinformation indicative of the force transmitted between the platform andthe stop.

An embodiment comprises a limb attaching the platform to the positioner,wherein at least part of the force is transmitted via the limb to theforce sensing system.

In an embodiment, the force sensing system is operationally coupled tothe limb.

An embodiment comprises a structure supporting the material receivingsurface, wherein the force sensing system is operationally coupled tothe structure.

In an embodiment, the force sensing system engages the structure to achassis.

In an embodiment, at least part of the force is transmitted via thestructure to the force sensing system.

In an embodiment, the structure comprises a window. The apparatus maycomprise a material solidifying radiation source configured toilluminate the material when so disposed with a material solidifyingradiation through the window.

In an embodiment, the control system is configured to operate thepositioner so that the distance information satisfies a distancecondition.

In an embodiment, the distance condition comprises that the distancebetween the platform and the material receiving surface indicated by thedistance information is within a predefined distance range.Alternatively, the distance condition comprises that the distancebetween the platform and the material receiving surface indicated by thedistance information is a predefined distance.

In an embodiment, the control system is configured to use the forceinformation to control the magnitude of the force. The control systemmay be configured to receive area information indicative of an area ofat least one section of the stereolithographic object and control themagnitude of the force using the area information. The control systemmay be configured to give non-zero weightings to each of a plurality ofareas of a plurality of sections of the stereolithographic object whencontrolling the magnitude of the force using the area information. Thenon-zero weightings may be determined using viscosity informationindicative of the viscosity of the material. The viscosity informationindicative of the viscosity of the material may comprise a viscositydistance.

In an embodiment, the force sensing system comprises a plurality offorce sensing elements that are spaced apart. The plurality of forcesensing elements may be spaced apart in at least one direction that isorthogonal to a normal to the material receiving surface. The pluralityof force sensing elements may be spaced apart in two directions that areeach orthogonal to a normal to the material receiving surface. The forceinformation may be indicative of a portion of the force sensed by eachof the plurality of force sensing elements. The control system may beconfigured to use the force information to determine a position on thematerial receiving surface that the force is applied to.

An embodiment comprises a flexible element comprising the materialreceiving surface.

In an embodiment, the flexible element forms at least part of a vesselconfigured to contain the material.

In an embodiment, the material for making the stereolithographic objectcomprises a liquid.

In an embodiment, the liquid is a sheet of liquid.

In an embodiment, the control system is configured to operate thematerial solidifying radiation source to a portion of the stereographicmaterial when so disposed to form a stereolithographic section of thestereolithographic object.

An embodiment comprises a material solidifying radiation manipulatorconfigured to manipulate radiation generated by the solidifying materialradiation source.

In an embodiment, the material solidifying radiation manipulator isconfigured to impart a spatial feature to the material solidifyingradiation.

In an embodiment, the radiation manipulator imparts a temporal featureto the material solidifying radiation.

In an embodiment, the control system is configured to control thepositioner to increase the distance of the platform and the materialreceiving surface after operating the solidifying material radiationsource.

In an embodiment, the material receiving surface is upwardly facing.

An embodiment is configured such that the material receiving surface ishorizontally orientated.

In an embodiment, the control system is configured to receiveinstructions for making the stereolithographic object.

In an embodiment, the instructions may comprise data indicative of aplurality of sections to be sequentially formed.

In an embodiment, the control system is configured to move thepositioner to a position such that the force is greater than when thecontrol system moves the positioner to a position for making a sectionof the stereolithographic object.

In an embodiment, the control system is configured to execute steps of amethod for displacing the material between the material receivingsurface and the platform, the method comprising the steps of:

-   -   operating the positioner to move the stereolithographic object        being made towards the material receiving surface;    -   operating the positioner to increase the deformation of the at        least one member until the magnitude of the force indicated by        the force information is at least one of equal to and greater        than a maximum force magnitude value; and    -   determining whether the distance information satisfies the        distance condition, and if so satisfied stop the positioner.

In an embodiment, if the distance condition is so satisfied, thepositioner is stopped at a position wherein the at least one member isnot deformed.

Disclosed herein is an apparatus for making a stereolithographic object.The apparatus comprises a platform for making the stereolithographicobject thereon and a material receiving surface for disposing thereon amaterial for making the stereolithographic object. The apparatuscomprises a positioner operationally coupled to at least one of theplatform and the material receiving surface, and operable to reduce thedistance between the platform and the material receiving surface. Theapparatus comprises a force sensing system comprising a plurality offorce sensing elements that are configured to generate force informationindicative of a portion of a force sensed by each of the plurality offorce sensing elements and transmitted between the platform and thematerial receiving surface by debris therebetween. The apparatuscomprises a processor configured to use the force information todetermine the position of the debris.

In an embodiment, the plurality of force sensing elements are spacedapart. The plurality of force sensing elements may be spaced apart in atleast one direction that is orthogonal to a normal to the materialreceiving surface. The force sensing system may comprise a plurality offorce sensing elements that are spaced apart in two directions that areeach orthogonal to a normal to the material receiving surface.

Disclosed herein is an apparatus for making a stereolithographic object.The apparatus comprises a platform for making the stereolithographicobject thereon and a vessel for disposing therein a material for makingthe stereolithographic object. The apparatus comprises a force sensingsystem configured to generate force information indicative of the weightof the material when so disposed. The apparatus comprises a processorthat determines when the force information satisfies a material weightcondition and if so generates a material weight condition signal.

In an embodiment, the force sensing system supports the vessel.

In an embodiment, the material weight condition is that the weight ofthe material indicated by the force information is one of equal to andless than a predefined material weight value.

Disclosed herein is a method for making a stereolithographic object. Themethod comprises the step of disposing a material for making the objectbetween a platform for making the object thereon and a materialreceiving surface. The method comprises the step of changing thedistance between the platform and the material receiving surface. Themethod comprises the step of generating force information indicative ofa force transmitted between the platform and the material receivingsurface. The method comprises the step of generating distanceinformation indicative of the distance between the platform and thematerial receiving surface using the force information.

An embodiment comprises controlling a change in the distance between theplatform and the material receiving surface using the force information.

An embodiment comprises controlling a reduction in the distance betweenthe platform and the material receiving surface using the forceinformation.

In an embodiment, the force comprises a material displacement force fordisplacing a portion of the material and so reduce the distance betweenthe platform and the material receiving surface.

In an embodiment, generating the distance information comprisescorrecting for a distance information error caused by deformationresulting from the force. The deformation may comprise deformation of amechanical linkage between the platform and the material receivingsurface.

An embodiment comprises using distance error correction information tocorrect the distance information error.

An embodiment comprises generating the distance error correctioninformation. Generating the distance error correction information maycomprise empirically generating the distance error correctioninformation. Empirically generating the distance error correctioninformation may comprise:

-   -   engaging the platform with a stop; and    -   generating force information indicative of a force transmitted        between the platform and the stop for each of a plurality of        positioner positions.

An embodiment comprises illuminating the material with a materialsolidifying radiation.

An embodiment comprises changing the distance between the platform andthe material receiving surface so that the distance informationsatisfies a distance condition. The distance condition may be that thedistance between the platform and the material receiving surfaceindicated by the distance information is within a predefined distancerange. The distance condition may alternatively comprise that thedistance between the platform and the material receiving surfaceindicated by the distance information is a predefined distance.

An embodiment comprises controlling the magnitude of the force using theforce information.

An embodiment comprises controlling the magnitude of the force usingarea information indicative of an area of at least one section of thestereolithographic object.

An embodiment comprises giving non-zero weightings to each of aplurality of areas of a plurality of sections of the stereolithographicobject when controlling the magnitude of the force using the areainformation. The non-zero weightings may be determined using viscosityinformation indicative of the viscosity of the material. The viscosityinformation indicative of the viscosity of the material may comprise aviscosity distance.

An embodiment comprises using force information indicative of a portionof the force sensed by each of a plurality of spaced apart force sensingelements to determine a position on the material receiving surface thatforce is applied to.

Disclosed herein is a method for locating the position of debris. Themethod comprises disposing a material for making the stereolithographicobject on a material receiving surface adjacent a platform for makingthe stereolithographic object thereon. The method comprises reducing thedistance between the platform and the material receiving surface suchthat the debris contacts the material receiving surface and theplatform. The method comprises generating force information indicativeof a portion of a force sensed by each of a plurality of force sensingelements that are spaced apart and transmitted between the platform andthe material receiving surface by the debris. The method comprisesdetermining the position of the debris using the force information.

In an embodiment, the plurality of force sensing elements are spacedapart.

In an embodiment, the plurality of force sensing elements are spacedapart in at least one direction that is orthogonal to a normal to thematerial receiving surface.

In an embodiment, the plurality of force sensing elements that arespaced apart in two directions that are each orthogonal to a normal tothe material receiving surface.

Disclosed herein is a method for monitoring consumption of a materialfor making a stereolithographic object. The method comprises disposing amaterial for making a stereolithographic object in a vessel adjacent aplatform for making a stereolithographic object thereon. The methodcomprises making the stereolithographic object on the platform and indoing so consuming the material disposed in the vessel. The methodcomprises generating force information indicative of the weight of thematerial in the vessel. The method comprises determining whether theforce information satisfies a material weight condition and if sosatisfied generate a material weight condition signal.

An embodiment comprises the steps of:

-   -   operating the positioner to move the stereolithographic object        being made towards the material receiving surface;    -   operating the positioner to increase the deformation of the        mechanical linkage until the magnitude of the force indicated by        the force information is at least one of equal to and greater        than a maximum force magnitude value; and    -   determining whether the distance information satisfies the        distance condition, and if so satisfied stop the positioner.

In an embodiment, if the distance condition is so satisfied, thepositioner is stopped at a position wherein the at least one member isnot deformed.

Disclosed herein is non-transitory processor readable tangible mediaincluding program instructions which when executed by a processor causesthe processor to perform a method disclosed above.

Disclosed herein is a computer program for instructing a processor,which when executed by the processor causes the processor to perform amethod disclosed above.

Any of the various features of each of the above disclosures, and of thevarious features of the embodiments described below, can be combined assuitable and desired.

BRIEF DESCRIPTION OF THE FIGURES

Embodiments will now be described by way of example only with referenceto the accompanying figures in which:

FIGS. 1 to 7 show schematic side elevation views of one embodiment of anapparatus for making a stereolithographic object.

FIG. 8 shows a schematic side elevation views of another embodiment ofan apparatus for making a stereolithographic object.

FIG. 9 shows an example architecture of schematic of a control system ofthe apparatus of FIGS. 1-8.

FIGS. 10 to 12 show schematic views of example radiation sources thatmay form part of a device for making a stereolithographic object.

FIG. 13 shows a graph of empirical data used to determine a viscositydistance value.

FIGS. 14 and 15 shows example flow charts of embodiments of methods.

FIGS. 16 and 17 show section views of embodiments of an apparatus formaking a stereolithographic object.

FIG. 18 shows a graph of empirical data relating force and deflection inan embodiment of an apparatus for making a stereolithographic object.

DESCRIPTION OF EMBODIMENTS

FIGS. 1 to 7 show schematic views of one embodiment of an apparatus formaking a stereolithographic object, the apparatus being generallyindicated by the numeral 100. In the context of this document, astereolithographic object is an object that has been made using astereolithographic process. Coordinate axes are shown in the figureswhere x and y are horizontally orientated and z is verticallyorientated.

The apparatus 100 comprises a platform 121 for making thestereolithographic object thereon. The apparatus 100 has a materialreceiving surface 102. In use, a material 104 for making thestereolithographic object is disposed between the material receivingsurface 102 and the platform 121. The apparatus 100 has a positioner 120operably coupled to at least one of the platform 121 and the materialreceiving surface 102. The positioner 120 is operable to change thedistance between the platform 121 and the material receiving surface102. The apparatus 100 comprises a force sensing system 105 configuredto generate force information indicative of a force transmitted betweenthe platform 121 and the material receiving surface 102. The apparatus100 comprises a control system 160. In this but not all embodiments, thecontrol system 100 is arranged to generate distance informationindicative of the distance between the platform 121 and the materialreceiving surface 102 using the force information.

Further features of this embodiment will now be disclosed. Otherembodiments may have any combination of the further features disclosed,or none of the further features disclosed. The control system 160 isconfigured to control the positioner 120 using the distance information.The positioner 120 can be so controlled to reduce the distance betweenthe platform 121 and the material receiving surface 102. The positioner120 can also be generally so controlled to increase the distance betweenthe platform 121 and the material receiving surface 102. The positioner120 is configured for linear motion along the plus and minusz-directions and is attached to a limb in the form of a bracket 123. Thepositioner 120 moves the platform 121 in the form of an invertedplatform on which the stereolithographic object 122 being made ismounted. Alternatively, the positioner 120 may be arranged to move thematerial receiving surface 102 or both the material receiving surface102 and the platform 121. During fabrication, the stereolithographicobject being made 122 is attached to the platform 121. The positioner120 positions the platform 121 and consequently the object being made122 relative to the material receiving surface 102, which is in this butnot all embodiments an upwardly facing surface.

The positioner 120 is controlled by the control system 160 to reduce thedistance between the platform 121 and the material receiving surface102. The control system 160 uses the distance information to control thepositioner 120. The control system 160 is configured to receiveinstructions for making the stereolithographic object in the form ofdata indicative of a plurality of sections (e.g. 124,125,129) to beformed sequentially by the apparatus 100. Each individually determinedsection may differ from another of the sections by, for example, theshape of their respective boundaries. Not every section needs to bedifferent, however. The control system 160 is configured to coordinateoperation of the positioner 120, a material solidifying radiation source116, and in some embodiments other parts, such that the plurality ofsections are sequentially formed in accordance with the receivedinstructions. The control system 160 comprises a processor.

In the context of this specification, a section is to be understood toencompass a slice of the stereolithographic object. A planar sectionencompasses a portion of the stereolithographic object located betweentwo parallel planes that intersect the stereolithographic object.Generally, but not necessarily, the sections formed are planar sections.

The control system 160 is configured to operate the positioner 120 sothat the distance information satisfies a distance condition. Thedistance condition comprises, in one mode of operation, that thedistance between the platform 121 and the material receiving surface 102indicated by the distance information is within a predefined distancerange. The predefined range may correspond to a distance between thestereolithographic object being made 122 and the material receivingsurface 102, being the thickness of one section of the object being made122, to within a tolerance of, for example ±5%. Alternatively, forexample, the distance condition comprises that the distance of theplatform 121 and the material receiving surface 102 indicated by thedistance information is a predefined distance.

The force is generated by the positioner 120 when, for example, thedistance between the platform 121 and the material receiving surface 102is reduced. As shown in FIG. 2, the stereolithographic object being made122 is moved into the material 104 by operation of the positioner 120,which displaces material 104 between the last formed section 125 (and sothe platform 121) and the material receiving surface 102. The forcecomprises a material displacement force when the stereolithographicobject being made 122 is moved through the material 104. The material104 is viscous and so resists being squeezed out of the gap between theobject being made 122 (and so the platform 121) and the materialreceiving surface 102. Consequently, a material displacement force isrequired to squeeze the material 104 out of the gap between the lastformed section 125 (and so the platform 121) and the material receivingsurface 102. The material displacement force, however, results in areactive force transmitted to at least one member 123, 124, 120, 156,130, 105, 510, 201, 101 of a mechanical linkage between the platform 121and the material receiving surface 102. The at least one member 123,124, 120, 156, 130, 105, 510, 201, 101 is deformed by the reaction tothe material displacement force. The deformation is an elasticdeformation. The reactive force pushes the platform 121 and the materialreceiving surface 102 apart. FIG. 2 shows the deformation of the limb123, the distal end 127 of which is deflected away from the materialreceiving surface 102 by a deflection distance d. The positioner 120 hasa linear encoder 131 that generates a positioner position valuecommunicated to the control system 160. The distance informationindicative of the distance between the platform 121 and the materialreceiving surface 102 can be estimated using the positioner positionvalue, however, an error is generally introduced by the deformation ofthe least one member 123, 124, 120, 156, 130, 105, 510, 201, 101. Thedeflection distance d is not easily directly measurable because it maybe of the order of 1 μm-200 μm, for example, and does not change thepositioner position value. Without further information and examples ofmethods described herein, the control system 160 would be unable toexactly determine when a layer of material 104 of the required thicknesshas been formed.

The control system 160 is configured to generate distance information bycorrecting for the distance information error caused by the deformation.The apparatus 100 comprises memory 240 which is part of, in this but notall embodiments, the control system 160. Stored in the memory 240 isdistance error correction information. The control system uses thedistance error correction information to correct the distanceinformation error. The deflection amount d is estimated by the controlsystem 160 using the force information generated by force sensing system105 and a two-column lookup table characterizing the relationshipbetween the force measured by the force sensing system 105 and thedeflection amount d. Alternatively, the lookup table may have athree-column lookup table, relating positioner position value, a valueof force information, and a value of distance information without thedeformation induced error. Alternatively, the distance error correctioninformation may be calculated by the control system 160 using amathematical function, for example a function describing a curve orstepped function. The distance error correction information maygenerally take any suitable and desired form.

The control system 160 is configured to execute a method for empiricallygenerating the distance error correction information, however the errorcorrection information may be alternatively generated from amathematical model. Alternatively, the distance error correctioninformation may be determined without use of apparatus 100 andsubsequently loaded into memory 240. The steps of one example of amethod for empirically generating the distance error correctioninformation is now disclosed. A step comprises engaging the platform 121with a stop 201 in the form of a material hardening radiationtransparent window. The material hardening radiation transparent window201 is in the form of a material hardening radiation transparent plate.A flexible element 101 in the form of a material hardening radiationtransparent sheet comprising the material receiving surface 102 isgenerally but not necessarily first removed (or alternatively the stopmay be flexible element 101 and the window 201). The stop may take analternative form and be temporarily introduced between the platform 121and the window 201. A step comprises generating force informationindicative of a force transmitted between the platform 121 and the stop201 for each of a plurality of positioner positions. Each of theplurality of positioner positions nominally position the platform 121 atdistances below the stop 201, resulting in deflections in the apparatus100 equivalent to the distances. The force information is generated bythe force sensing system 105, however it may alternatively be generatedusing a force sensor removably disposed intermediate the platform 121and the stop 201, for example. An example graph of force versusdeflection data experimentally obtained from an embodiment is shown inFIG. 18. A deflection of around 27 microns produces a force of around 1kg in the force sensing system, and a deflection of around 170 micronsproduces a force of around 8 kg. During typical operation ofembodiments, the forces may be in the order of 10 kg while the requiredlayer accuracy is of the order of 10 microns. Thus, the deflectionsencountered in an embodiment may be an order of magnitude greater thanthe precision required. Such deflections would detrimentally impact thefidelity of parts built by the apparatus, if counteracting measures werenot taken.

The advantage of the apparatus 100 determining the distance errorcorrection information directly is that it incorporates the deflectionsin the entire mechanical linkage between the build platform 121 and thematerial solidifying radiation transparent glass plate 201. It may alsoaccount for different deformations experienced by different examples ofapparatus 100 due to manufacturing imperfections. When the materialhardening radiation is 385 nm wavelength light, for example, the window201 may comprise a 6 mm thick plate of fused silica. The edges of thewindow 201 may be beveled, or even rounded, to reduce the risk of ascratch or other mark being made on the underside surface 103 of theflexible element 101.

During operation, the deflection amount d is estimated from the forceindicated by the force information and the lookup table. Interpolationof lookup table data is generally but not necessarily used.Alternatively, the force information may be first rounded to the samelevel of precision as the force data in the look-up table, or the lookup table may be stepped through until the nearest force value entry isidentified. The estimation of deflection amount d begins by measuring abaseline signal from the force sending system 105 while thestereolithographic object being made 122 is positioned a suitabledistance away from the material receiving surface 102 such that it doesnot exert any significant (or any) pressure on the material receivingsurface 102. A typical distance may be around 2 mm, wherein thestereolithographic object 122 being made is immersed in the material104. The baseline measurement may also allow the weight of the material104 in the material vessel 108 and the weight of material displaced bythe immersed object 122 to be subtracted from the force information, ifnecessary. Then, as the platform 121 approaches the material 104, theforce sensed by the force sensing system 105 minus the baseline forcegives a measure of the total reactive force. The reactive force is usedas input to the lookup table to estimate the corresponding deformation(e.g. deflection) amount. The actual distance of the platform 121 fromthe material receiving surface 102 is then estimated as the position ofthe positioner 121 plus the deflection amount from the lookup table. Ifthe motion is away from the material receiving surface 102 and thereactive force is negative, the deflection would change sign and theestimated position may be the position of the positioner minus theestimated deformation.

The estimated position allows the apparatus 100 to be controlled moreaccurately as it provides a means for determining, with high precision,the thickness of a layer of material 104 between the stereolithographicobject 122 being made and the material receiving surface 102. This mayresult in a stereolithographic object 122 that may better reflectinstructions received by the control system 160 that specify thestereolithographic object 122.

The force sensing system 105 comprises a plurality of force sensingelements in the form of four load cells 411, 412, 413 and 414. There maybe more or less force sensing elements in other embodiments, and in oneembodiment there is a single force sensing element. The plurality offorce sensing elements may be spaced apart in at least one direction,and in this embodiment two directions (x and y) that are orthogonal to anormal (z direction) to the material receiving surface. The load cells411, 412, 413 and 414 may be piezoelectric, or generally any suitabletype, however in the present embodiment they are resistive load cells inthe form of strain gauges that have an electrical resistance between twoelectrical terminals that are proportional to a magnitude of a forceapplied normal to a force receiving surface. The force sensing system105 comprises electronic circuitry that measures the resistance of theload cells and generates a digital force signal indicative of the forcethat is communicated to the control system 160.

The force sensing system 105 engages the structure 510 to a chassis 130,to which the positioner 120 is attached. Engagement may be viaattachment with, for example, fasteners or adhesive, or by, as in thepresent embodiment, providing a seat for the structure so that thestructure 510 is suspended on the force sensing elements 411, 412, 413,414. In turn, the force sensing system 105 may be attached to thechassis 130 or received in a seat provided by the chassis. Other formsof engagement may be provided. The mechanical linkage comprises, in thisembodiment, the flexible element 101, the structure 510, the forcesensing system 105, the chassis 130, bracket 156, the positioner 120,and limb 123. Any one or more of these members may deform. Consequently,the force sensing system 105 is operationally coupled to these membersof the linkage and the force is transmitted by at least some of thesemembers to the force sensing system 105.

The control system 160 is configured to use the force information tocontrol the magnitude of the force to, for example, reduce the timetaken to make the stereolithographic object 122. It may take aninconveniently long time for the material 104 between the materialreceiving surface 102 and the object being made 121 to be displacedduring reduction of the distance between the platform 121 and thematerial receiving surface 102, and the mechanical linkage to return toits non-deformed shape as the force decreases towards zero. This timemay be in the order of several minutes depending on one or moreproperties of the material (e.g. viscosity), the area of the previouslyformed section or sections of the stereolithographic object 122 beingmade, the rigidity of the mechanical linkage, and the section thicknesswhich is desired to be formed. This generally may add to the fabricationtime and diminish productivity.

The force applied by the limb 123 to displace (“squeeze out”) thematerial 104 between the stereolithographic object being made 122 andmaterial receiving surface 102 decreases as the mechanical linkagereturns to its non-deformed shape. This is similar to the decrease inforce exerted by a spring as it returns to its non-deformed state. Forexample, the deflection d of the limb 123 may asymptotically approachzero. This is a contributing factor to what may be an inconvenientlylong amount of time to displace the material 104.

Additional force may be applied to increase the rate the material 104 isdisplaced. The control system 160 is configured to move the positioner120 to a position such that the force is greater than when the controlsystem 160 moves the positioner 120 to a position for making a sectionof the stereolithographic object 122—that is, when the distance betweenthe stereolithographic object 122 being made and the material receivingsurface 102 is the thickness of one section of the stereolithographicobject 122 in the absence of apparatus deflections. The control system160 operates the positioner 120 such that it takes a target positionwhich would be below the target position necessary to achieve therequired section thickness in the absence of apparatus deflections. Thissituation is shown in FIG. 3. In this case, the deflection of thebracket 123 is increased from d to d″. The greater applied force resultsin greater deformation.

Accordingly, the control system 160 is configured to execute steps of anexample of a method for displacing the material 104 between the materialreceiving surface 102 and the platform 121. In a step, the controlsystem 160 operates the positioner 120 to move platform 121—and so thestereolithographic object 122 being made—towards the material receivingsurface 102. The control system 160 operates the positioner 120 toincrease the deformation of the at least one member 123,130 until themagnitude of the force indicated by the force information is at leastone of equal to and greater than a maximum force magnitude value. Thecontrol system 160 determines whether the distance information satisfiesa distance condition, and if so satisfied stops the positioner. In thisbut not all embodiments, the control system 160 may reverse the motionof the positioner and subsequently stop the positioner at a positionwherein the at least one member is not deformed. The distance conditioncomprises, in one mode of operation, that the distance of the platformand the material receiving surface indicated by the distance informationis within a predefined distance range. The predefined range maycorrespond to a distance between the stereolithographic object beingmade 122 and the material receiving surface 102 being the thickness ofone section of the object being made 122, to within a tolerance.Alternatively, for example, the distance condition comprises that thedistance of the platform and the material receiving surface indicated bythe distance information is a predefined distance, for example. FIG. 14shows a flow chart indicated by the numeral 300 for an embodiment of themethod.

During execution of the steps of the example method for displacing thematerial 104 between the material receiving surface 102 and the platform121, the positioner 120 may have moved the positioner such that thedistance between the platform and the material receiving surface is lessthan a predefined distance, for example less than the lower end of thepredefined distance range as described above or another predefineddistance. The control system 160 is configured to determine whether thedistance indicated by the distance information is less than a predefinedminimum distance, and if so increase the distance so that the distancecondition is satisfied.

When the desired distance between the platform and the materialreceiving surface 102 is achieved, the positioner 120 is moved to removethe deformation, as shown in FIG. 4. The stereolithographic object 122being made is positioned at one section-thickness above the materialreceiving surface 102 when the flexible element 101 is in contact withthe reference surface 202.

Apparatus 100 was constructed by the applicant. A rectangular sectionhaving dimensions 125 mm by 70 mm was brought to within 50 μm (thedistance condition) of the material receiving surface 102 covered with aphotohardenable liquid material 104 having a viscosity of around 2500centipoise. When the positioner was moved to a final position and themechanical linkage deformed, with the initial displacement force of 0.5kg (limited by the stiffness of the apparatus), it took about 6.8minutes for the deformation to relax and the distance condition to besatisfied. In comparison, when the example of the method for displacingthe material 104 was employed, the same distance condition was attainedin 1.2 minutes by applying a consistent force of 8.5 kg. When thedistance condition was attained in this example, the apparatus'deflection was estimated to be 185 microns and the positioner was thenmoved upwards by 185 microns to relieve the deflection. The decrease inthe time taken to satisfy the distance condition may improve theproductivity of the apparatus whilst not sacrificing accuracy.

In some embodiments the control system 160 may utilize force informationto control the target position of the positioner 120 and thereby controlthe force applied to the stereolithographic object 122 being made. Insome embodiments it may be desirable to control the force applied to thestereolithographic object 122 being made, for example when thestereolithographic object 122 being made is fragile, to prevent damageto the stereolithographic object 122 being made. In other embodiments itmay be of greater utility for the control system 160 to control thepressure applied to the stereolithographic object 122 being made, thepressure being the ratio of the applied force to the cross-sectionalarea of the section being formed. In this manner, the applied force maybe different when forming different sections of the object 122.

The steps of an example of a method for controlling the force is nowdescribed. In a step, a target position is set by the control system160. In a step, the positioner moves the stereolithographic object beingmade 122 towards the material reference surface 102 while monitoring theforce information. In a step, the movement of the positioner 121 isstopped if the force indicated by the force information exceeds themaximum allowable force. In a step, the motion towards the targetposition is continued when the force indicated by the force informationis less than the maximum allowable force. When the target position isreached the movement is stopped. FIG. 15 is a flow chart of anembodiment of the method.

In determining the appropriate force to apply when forming a section ofthe stereolithographic object 122, the sections formed prior to the lastformed section 125 of the stereolithographic object 122 may influencethe force used to bring the object adjacent to the surface 102. Thecontrol system 160 is configured to receive area information indicativeof an area of at least one section 124,129 of the stereolithographicobject 122 and control the magnitude of the force using the areainformation. When determining the magnitude of the force to apply, afunction of the cross-sectional areas of the previous sections may beused. For example, it may be appropriate to use a weighted averagecross-sectional area of the 10 previous sections as the basis area onwhich to calculate the applied force from the allowed pressure to beapplied to the object. Alternatively, a finite-impulse-response filter,an infinite-impulse-response filter, or any other suitable type offilter may be applied to the previous cross-sectional area data todetermine the basis area. The control system 160 is configured to givenon-zero weightings to each of a plurality of areas of a plurality ofsections of the stereolithographic object when controlling the magnitudeof the force using the area information. The number of previous sectionsappropriate to use in the weighted calculation may depend on theviscosity of the resin and the distance over which a viscous force maybe exerted by a surface brought adjacent to another in the presence ofthe material 104. In the present apparatus we refer to this distance asthe viscosity distance of a resin. Viscosity distance can be calculatedexperimentally on the apparatus 100 using the positioner 120 and forcesensing system 105. The positioner 120 is controlled for bringing theplatform 121 to a defined distance from the sheet. This produces amomentary force in the force sensing system as the viscous material isdisplaced, which resolves (that is, reduces towards zero) after a periodof time. The positioner then moves the build platform towards the sheetin set increments, for example 0.1 mm increments, in each case thenpausing to measure the time taken for the increased force sensed exertedon the force sensing system 105 to dissipate due to material 104 flowingout of the gap between the two surfaces. An example curve of dissipationtime versus distance from the sheet is shown in FIG. 13. The viscositydistance is defined as the distance at which the deformation inducedforces relax in a given time. For the example curve shown in FIG. 13 ifwe define the resolving time as 5 seconds, which may be a reasonablewaiting time between sections during a stereolithographic process, theviscosity distance would be defined as approximately 0.6 mm. If anobject having cross sectional area equal to that of the platform 121 isbrought to within 0.6 mm of a surface in the presence of this material104, the force exerted on the surface would resolve (that is, fall toaround zero) in around 5 seconds. This gives a practical unit of measureof the viscosity of the material 104 and the distance over which priorsections could exert a significant force against the force sensingsystem 105, specifically the force sensing elements 411, 412, 413 414.The non-zero weightings are determined by the control system 160 usingviscosity information indicative of the viscosity of the material, andin this but not all embodiments in the form of the viscosity distance.The measurement of the viscosity distance value may be automated by thecontrol system. Taken together, force sensing elements 411,412,413,414may operate as a position-sensitive detector which can measure theposition and magnitude of forces applied to the mounting platform in twodimensions. For example, the plurality of force sensing elements411,412, 413, 414 permits positional information indicative of thelocation of solid debris between the object being made 122 and thematerial receiving surface 102. The force information is indicative of aportion of the force sensed by each of the plurality of force sensingelements 411, 412, 413, 414. If the forces applied to the four forcesensing elements are F1, F2, F3, F4, and the force sensing elements arelocated at corresponding positions coordinates L1, L3, L3, L4—which areexpressed as vectors (x,y)—where the origin is at the geometric centreof mass of the force sensing elements' locations, and the sum of theforces sensed by the plurality of force sensing elements 411,412,413,414is F, the location of the debris detected is estimated as the weightedposition using the following example of a force centre function:(x _(c) ,y _(c))=L1*F1/F+L2*F2/F+L3*F3/F+L4*F4/FThe control system 160 is configured to calculate (x_(c),y_(c)) usingthe above force centre function and the force information to determine aposition on the material receiving surface 102 that the force is appliedto.

The control system 160 of apparatus 100 may be configured to detecthigher than expected forces during fabrication, for example as a resultof the debris. In such circumstances, the fabrication may be halted toprevent damage to the apparatus. The control system is configured togenerate a debris alert for a user. The alerted user may be providedwith a two-dimensional position of the debris on the material receivingsurface 102 generated by the control system 160. This may be ofparticular utility when debris is obscured by opaque photohardenablematerial 104. The notified user may remove the debris, having beenalerted of its presence. The fabrication process may then continue,which may prevent the process from being restarted, wasting time andmaterial 104.

A processor within the control system 160 uses the force information todetermine whether the force indicated satisfies a material weightcondition. When there are no material displacement forces applied, theforce is indicative of the weight of the structure 510, the materialvessel 108 and the material 104 therein. The weight of the material 104is determined by the processor 160 by subtracting an empty-vesselreading. The processor generates an alert if it determines that theweight of the material satisfies a material weight condition, forexample the weight of the material is less than a minimum materialweight value. The control system 160 is configured to pause thefabrication process when it is determined that the material 104 in thevessel 108 is insufficient and needs replenishment. After replenishment,the fabrication process may be continued by the control system 160 whenit determines a sufficient material weight condition is satisfied, forexample, the material exceeds a minimum material weight value. This mayreduce the likelihood of failed builds due to insufficient material.

The use of a plurality of force sensing elements 411,412,413,414 mayprovide redundant information. Forces may be located in two dimensionswith only three suitably positioned force sensing elements. In otherembodiments, a single force sensing element suitably mounted below themounting platform 510 may be sufficient to provide force onlyinformation. Redundant information may allow for more precisemeasurement through noise filtering. Using a plurality of force sensingelements below the mounting platform 510 also provides the opportunityto mount the mounting platform without cantilevering, which improves therobustness and stiffness of the apparatus 100. This is shown in FIGS.1-7, and also FIG. 16 which shows section-A of the apparatus fromFIG. 1. A section of an alternative embodiment is shown in FIG. 17 whichemploys a pair of force sensing elements 415 and 416 spaced apart tosupport the mounting platform 510. In this embodiment, the mountingplatform 510 is beneficially mounted without cantilevering andone-dimensional spatial information regarding the distribution of forcescan be derived from the load cell readings using the methods describedherein.

Further details of apparatus 100 will now be described, which may beshared by other embodiments.

The apparatus 100 has a flexible element 101 in the form of asubstantially transparent sheet with upward facing material receivingsurface 102, however in other embodiments the material receiving surface102 may be downward facing. The material 104 is in the form of a layerof photohardenable liquid 104 disposed on the material receiving surface102 and that hardens when exposed to a material solidifying radiation.The material solidifying radiation may be visible or invisible light(ultraviolet light, for example). Example wavelengths of suitable lightinclude 355 nm, 385 nm, and 405 nm. In some embodiments, radiationsources other than light may be used. For example, the radiation sourcemay be ionizing or non-ionizing radiation.

The photohardenable liquid may comprise a mixture of acrylate monomersand oligomers, photoinitiators, colourants and stabilizers such that themixture polymerizes when exposed to suitable light. Example liquidsinclude Somos NEXT from DSM Somos, USA, and KZ-1860-CL from AlliedPhotoPolymers, USA. In alternative embodiments, the material maycomprise a powder such as a fluidized polymer powder, or a paste. Anysuitable material may be used. Flexible element 101 may possessanti-stick properties in relation to the material 104 when it is curedin contact with the sheet. Suitable materials for sheet 101 include FEPfluoropolymer film manufactured by Du Pont, USA. The film may be ofaround 125 micrometers thickness, but may be thicker or thinner asappropriate. The sheets are flexible but may not be particularlyelastic, having a Young's modulus of around 560 MPa. Generally, but notnecessarily, a Young's modulus of between 100 and 1000 MPa may besuitable. Other examples of suitable materials include PFA fluoropolymerfilm and Teflon AF film, also manufactured by Du Pont. Still otherexamples of suitable sheet materials are silicone, polyethylene film,polyethylene terephthalate film, and cellulose acetate film. Generally,any suitable material may be used for the element.

In this embodiment, the flexible element 101 is not backed by anothermaterial or layer, and is homogeneous, that is has a uniform structureand composition throughout. In other embodiments the sheet may have amulti-laminate construction. For example, the sheet may comprise a layerof silicone bonded to a polyester film, the film providing a highYoung's modulus and the silicone providing a superior nonstick surfacein relation to the photohardenable material 104. Other materials orlaminates of different materials may alternatively be used.

The flexible element 101 and side walls 106 form a vessel 108 in theform of a trough or dish for containing the material 104. The vessel 108may have a volume sufficient to hold enough liquid to build an entirestereolithographic object without being replenished. Optionally, aconduit may connect the vessel and a supply of the material to replenishthe material as it is consumed. The flexible element 101 forms the baseof the vessel. The vessel 108 and material 104 contained therein can beeasily removed from the apparatus and replaced with another vessel andother material, thus providing a convenient means for replacing damagedvessels or making objects from different materials.

A reference surface 202 additionally shapes the flexible element 101 tohave it adopt a flat configuration (or any desired configuration, forexample a curved configuration) or form while excess photohardenableliquid 104 is forced out of the gap between the previously hardenedsections 122 and the flexible element 101. Support of the flexibleelement 101 by the reference surface 202 may allow for flat sections ofconsistent and precise thicknesses to be formed. In alternativeembodiments, the vessel 108 may incorporate the plate 201, flexibleelement 101, and side walls 106 to form a unitary construction withrigid base which may be removable from the apparatus.

The thickness of one section is typically in the range of 10 micrometersto 250 micrometers, but it may be less if particularly fine fabricationresolution is required, and greater if a relatively coarse fabricationresolution is required.

The apparatus 100 has member 301 that supports the flexible elementaround a perimeter of the transparent plate 201 having an uppermostreference surface 202. The underside of the flexible element 101 isbiased towards member 301 with biasing elements in the form of springelements 194,195 which causes the flexible element 101 to be tensionedin both the x and y directions. The reference surface 202 is positionedbelow the flexible element 101 which may prevent it from sagging. Insome embodiments the reference surface 202 may be adjacent the flexibleelement 101.

The apparatus 100 is configured such that in use the flexible element101 is horizontally orientated. The chassis 130 has attached feet132,133 configured to support the chassis 130 above a surface such as abench, and the flexible element 101 is mounted relative to the chassis130 so that when the chassis 130 is so supported the flexible element130 has a horizontal orientation. In other embodiments, the materialreceiving surface 102 may be inclined at up to 45 degrees to thehorizontal (that is, the surface is upwardly facing), provided that thevessel walls 106 are sufficiently high to contain the material 104.Mounting brackets 152,154,156,158 may be used to ensure that apparatuscomponents are maintained in their correct position and orientationrelative to the chassis 130. A mounting platform 510 may serve to mountapparatus components and form a fluid-tight division between the upperand lower regions of the apparatus 100 to prevent ingress of any spilledmaterial 104 which may damage delicate components.

The material solidifying radiation source 116 comprises a light source,and may be activated by the control system 160 so that it emitsspatially and/or structured light 118 capable of selectively hardeningareas of the material 104 to form a section of the stereolithographicobject 122. Material solidifying radiation source 116 may, for example,incorporate a light manipulator such as an image projection systemdepicted in FIG. 10 and generally indicated with the numeral 116 a,comprising light source 161 emitting light 162, relay optics 163,turning prism 164, spatial light modulator 165 controllable by controlsystem 168, and projection lens 166. Alternatively, material solidifyingradiation source 116 may be a light beam scanning apparatus depicted inFIG. 11 and generally indicated by the numeral 116 b, comprising a lasersource 171 emitting light 172 of wavelength of around 350 nm, forexample, collimating and/or focusing optics 173, scanning mirror 174whose rotation is controllable in one or more axes by mirror controller178, optionally a second controllable mirror not shown in the figure,and optionally a projection lens 175 such as an F-Theta lens. Controlsystem 160 can be configured to scan the mirror 174 (coordinated with asecond mirror, if present) in a raster scanning mode, or alternativelyin a vector scanning mode. FIG. 12 shows a second type of beam scanningapparatus generally indicated by the numeral 116 c comprising a lasersource 181 emitting light 182, collimating and/or focusing optics 183,polygon mirror 184 rotatable around an axis 185 and controllable bycontroller 188, and optionally a projection lens 186 such as an F-Thetalens. As the apparatus of 116 c may only scan light in the y-axisaccording to the coordinate system shown in FIG. 12, the apparatusresides on a translation stage 187 which can move the apparatus in thex-direction, enabling the projected light to address locations in the xand y dimensions. The translation stage may comprise any one or more oflinear motors, drive belts, stepper motors, rack and pinionarrangements, for example, or generally any suitable components arrangedto provide translation. Apparatus 116 c is suitable for operating in araster scanning mode. The light source may, in some embodiments,comprise an incandescent light or light emitting diode, for example. Anysuitable light source may be used.

The positioner 120 may comprise any one or more of linear motors, drivebelts, stepper motors, rack and pinion arrangements, for example, orgenerally any suitable components arranged to provide linear motion. Inthe present embodiment the positioner comprises a linear actuator in theform of a ball-screw linear stage driven by a stepper motor, a carriagemoved by the linear actuator and a rail orientated in the z directionalong which the carriage travels. The limb 123 is attached to thecarriage. The positioner may have a dedicated stepper motor controller,as in the present embodiment, however in other embodiments the controlsystem 160 may control the stepper motor. The carriage can be movedalong the rail to the positioner position value

The positioner 120, the light source, force sensing system 510 andpossibly other parts of the apparatus 100 may be in communication withand may be controlled by the control system 160 to coordinate theapparatus 100 to make the object. These and other components may beconnected by wires, cables, wireless, or any other suitable means. Inthis embodiment, the control system may have a processor 220 in the formof a processor unit, schematically illustrated in FIG. 9. The processorunit 220 may include a suitable logic device 250 such as, or similar to,the INTEL PENTIUM, ARM processor, or a suitably configured fieldprogrammable gate array (FPGA), connected over a bus 280 to a randomaccess memory 240 of around 100 Mb and a non-volatile memory such as ahard disk drive 260 or solid state non-volatile memory having a capacityof around 1 Gb. The processor has input/output interfaces 270 such as auniversal serial bus and a possible human machine interface 230 e.g.mouse, keyboard, display etc. Device components may be controlled usingcommercially available machine-to-machine interfaces such as LAB VIEWsoftware together with associated hardware recommended by the commercialinterface provider installed on the processor unit 220, over USB orRS-232 or TCP/IP links, for example. Alternatively, custom driversoftware may be written for improved performance together with customprinted circuit boards. Alternatively, the processor unit 220 maycomprise an embedded system, or a microcontroller.

In this embodiment, the control system 160 is in communication withanother processor which is adapted for determining instructions and/orinformation for the device. In alternative embodiments, the processorsare the same processor. An example of another processing unit comprisesa logic device such as, or similar to, the INTEL PENTIUM or a suitablyconfigured field programmable gate array (FPGA), connected over a bus toa random access memory of around 4 Gb and a non-volatile memory of suchas a hard disk drive or solid state non-volatile memory having acapacity of around 1 Tb. Generally, the configuration may be similar oridentical to that shown in FIG. 9. The processor has a receiver such asa USB port (or Internet connection, for example) for receivinginformation representing a solid object, stored on a USB FLASH device,for example. The information may be encoded in a file generated by aComputer Aided Design (CAD) program, the information specifying thegeometry of the object. The microprocessor runs a decomposer programimplementing an algorithm that decomposes (or transforms) theinformation into data indicative of a plurality of sections to be formedsequentially by the device, the material being used to make the solidobject. The program may have been installed onto the processor fromtangible media such as a DVD or USB memory stick, for example, thatstored the program. In an alternative embodiment, the decomposer may bea dedicated hardware unit. A series of sections through the object aredetermined, each section corresponding to a solid section to be formed.The sections may then be further processed to represent the geometry ofeach section as a rasterised bitmap. The sections or bitmaps may thenused to control the device.

FIGS. 1 to 7 taken in sequence are indicative of an embodiment of amethod for making an object. The method forms a new section of thestereolithographic object 122 and non-destructively separates it fromthe flexible element 101. In FIG. 1, the earlier formed plurality ofsection of the stereolithographic object 122 are spaced apart from theflexible element 101. In FIG. 2, positioner 120 lowers thestereolithographic object 122 being made towards the flexible element101. As the stereolithographic object 122 approaches the sheet, thematerial 104 is squeezed out of the gap between the stereolithographicobject 122 being made and the flexible element 101. In FIG. 3, thematerial displacement force is controlled and increased which may resultin deflection of the apparatus. As shown in FIG. 4, the positioner 120is reversed when the desired distance between the material receivingsurface 102 and the stereolithographic object 122 being made is reachedso that deflected is remove. Next, as shown in FIG. 5, materialsolidifying radiation 118 having spatial features in accordance with thesectional geometry of the object being made is emitted from light source116 to selectively solidify regions of the layer of material 104 incontact with the previously formed sections 122 to form a new hardenedsection 124. Next, as shown in FIG. 6, positioner 120 is engaged toraise the previously formed sections 122 and newly formed section 124,causing the flexible element 101 to stretch and distort. As the flexibleelement 101 is pulled away from the reference surface 202, once theangle between the flexible element 101 and stereolithographic object 122being made is sufficiently large, the flexible element 101 will peelaway from the newly formed section 124 and the apparatus 100 is readyfor the process to start again, as shown in FIG. 7. Repeating thissequence of actions enables a multi-laminate object to be fabricatedsection by section.

FIG. 8 shows another embodiment of an apparatus 200 for making astereolithographic object, wherein parts similar and/or identical inform and/or function to the apparatus 100 are similarly numbered.Apparatus 100 has only one force sensing element 415 between theplatform 121 and limb 123. The disclosure herein with reference toapparatus 100 also applies to apparatus 200, except for reference toplural force sensing elements, the location of the plural force sensingelements, and the functions that require plural force sensing elementsand their location may enable. The mounting platform 510 in thisembodiment is supported by mounting brackets 152,154. In alternativeembodiments, a plurality of force sensing elements may be configuredbetween the platform 121 and limb 123.

Embodiments described herein may be used to make a stereolithographicobject of generally any shape or size, including jewelry such as rings,prototype car components, micro-components for precision machines,models for investment casting, rapid prototypes, dental models, hearingaids, models of anatomical and other objects, circuit boards andarchitectural or design features for a building. The stereolithographicobject may, for example, be rigid or resilient. It may have one or morehollows or voids, such as that of a cup or tennis ball, for example.

Now the embodiments have been described, it will be appreciated thatsome embodiments of the invention may have some of the followingadvantages:

-   -   The distance between the platform and the material receiving        surface may be determined even in the presence of apparatus        deformation caused by material displacement forces.    -   The magnitude of the material displacement force may be safely        increased, which may reduce the time taken to make the        stereolithographic object.    -   The material displacement force applied may be controlled        independently of apparatus rigidity, which may enable cheaper        and less rigid embodiments to operate with similar performance        as more expense and rigid embodiments.    -   The amount of material remaining in the material vessel may be        measured, permitting the control system to pause the build        process when the material vessel requires replenishment.    -   Collisions with debris in the material liquid may be detected        preventing damage from occurring to the apparatus. The location        of the debris in the build envelope may be determined from the        measured forces.    -   The use of a flexible element may reduce the risk of damage to        the section and/or the stereolithographic object being made.

Variations and/or modifications may be made to the embodiments describedwithout departing from the spirit or ambit of the invention. While inthe present embodiment the material receiving surface is of a flexibleelement in the form of a sheet, the material receiving surface may be ofan inflexible part. Apparatus 100 and 200 may alternatively utilizevessels equivalent to 108 with rigid bottoms. For example, the vessel108 may incorporate both the side walls 106, sheet 101 and window 201.The vessel 108 may comprise a glass bottom coated with silicone or alayer of fluoropolymer such as those mentioned above to impartanti-stick properties. The flexible element 101 may not be a sheet, butrather may be wedged. The upwardly or downwardly facing surface of theflexible element 101 may be textured. The upward facing surface of thereference plate may be textured. The present embodiments are, therefore,to be considered in all respects as illustrative and not restrictive.Reference to a feature disclosed herein does not mean that allembodiments must include the feature.

Prior art, if any, described herein is not to be taken as an admissionthat the prior art forms part of the common general knowledge in anyjurisdiction.

In the claims which follow and in the preceding description of theinvention, except where the context requires otherwise due to expresslanguage or necessary implication, the word “comprise” or variationssuch as “comprises” or “comprising” is used in an inclusive sense, thatis to specify the presence of the stated features but not to precludethe presence or addition of further features in various embodiments ofthe invention.

The invention claimed is:
 1. An apparatus for making astereolithographic object, the apparatus comprising: a platform formaking the stereolithographic object thereon and a material receivingsurface spaced apart by a distance from the platform, wherein when theapparatus is in use a material for making the stereolithographic objectis disposed between the platform and the material receiving surface; apositioner operationally coupled to at least one of the platform and thematerial receiving surface, and operable to change the distance betweenthe platform and the material receiving surface; a control systemarranged to operate the positioner to move in a first direction adistance greater than required to achieve a target distance between theplatform and the material receiving surface, whereby the material whenso received is displaced by a generated material displacement force andthe distance between the platform and the material receiving surfacechanges, the control system being arranged to operate the positioner tomove in a second direction that is opposite the first direction torelieve the material displacement force and inhibit the change in thedistance between the platform and the material receiving surface; aforce sensing system configured to generate force information indicativeof a force transmitted between the platform and the material receivingsurface, wherein the control system is arranged to use the forceinformation to determine distance information indicative of the distancebetween the platform and the material receiving surface; and at leastone deformable member operationally coupled to the platform andcomprising a mechanical linkage between the platform and the materialreceiving surface, wherein the force sensing system is operationallycoupled to the at least one deformable member.
 2. The apparatus of claim1 wherein the control system is configured to operate the positioner toinhibit the change in the distance between the platform and the materialreceiving surface when the distance satisfies a distance condition. 3.The apparatus of claim 2 wherein the distance condition is that thedistance between the platform and the material receiving surface is apredefined distance.
 4. The apparatus of claim 1 further comprising astructure supporting the material receiving surface, wherein the forcesensing system is operationally coupled to the structure.
 5. Theapparatus of claim 4 wherein the structure comprises a window, andcomprising a material solidifying radiation source configured toilluminate the material when so disposed with a material solidifyingradiation through the window.